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Hello, the first thing to check when you come across the MCU error message is to make sure you are not having any USB connectivity issues, please refer to this helpful article to troubleshoot your connection taking into respect the operating system you are on (Windows, Mac, Linux): https://support.ledger.com/hc/articles/115005165269-Fix-USB-connection-issues-with-Ledger-Live?support=true
If issue persists after that, please check out the steps here as well: https://support.ledger.com/hc/en-us/articles/4405497803153-MCU-firmware-is-not-genuine?support=true
If nothing works, do not hesitate to open a ticket by pressing on the „Contact us“ button here: support.ledger.com/hc/en-us